The present invention deals with a device and a process for preventing the breaking of a metal wire passing between two guide elements. Such a device proves particularly useful in the field of electroerosion, for cutting machines equipped with a wire electrode.
Actually the wire electrode of a machine for cutting by electroerosion is subjected to great wear during its passage through the machining zone. This wear is due to the intense sparking undergone by the wire in this zone and likewise to friction on the guide and the power lead contact located upstream, this effect being increased further by the fact of heating up of the wire due to the machining electric current which passes through it and by the pull applied to it.
The surface of the wire becomes more and more uneven, which leads to an increase in the friction on the guide and the power lead contact located downstream. Moreover, its cross-section changes asymmetrically from one point to another of its length. Generally this involes a thinning of the wire, which increases its temperature rise by the Joule effect and which results, as well as the change in its surface condition, in a reduction of its mechanical resistance. As described in British Pat. No. 2,045,671, these local reductions in diameter are accompanied by excrescences which appear on their edges and sometimes greatly exceed the original cross-section of the wire. These excrescences "hang" on the wire guide downstream and induce lateral displacements of the wire, which increase the danger of short circuits and thus, for both of these reasons, increase the probability of breaking of the wire.
For a long time, by examining the wire coming out downstream from the machining zone, the practitioners of electroerosion have known how to predict a probable break in the wire by observing the increase in its unevenness and in the reduction in its diameter which are generally recognizable with the naked eye. It is known that when the wire takes on a certain appearance, there is reason to reduce the frequency of the electrical discharges so as to avoid the break.
This roughness of the wire increases during the machining, even though the wire passing into the machining zone is constantly renewed and the machining parameters remain constant and had been chosen at the start so as to obtain a substantial erosion of the piece and a low erosion of the wire. Indeed, the machining conditions deteriorate in the course of time in an unpredictable manner. Thus, filters may become plugged up, pumps may become less effective, thus changing the characteristics of the dielectric fluid in which the piece to be machined and the wire electrode are bathed. Leaks may occur which lower the effectiveness of the nozzles injecting the machining fluid, which results in a loss in pressure and thus a poor removal of the metal particles eroded and a poor cooling of the wire electrode. Thus it is desirable to alleviate this deterioration of the machining and this abnormal and irregular wear on the wire.
This is why it has been attempted to detect automatically and less empirically the state of deterioration of the wire, making it possible to detect its imminent break by measuring the variation in a physical characteristic of the wire following its passage through the machining zone. Thus, some of the known processes make use of monitoring the wear on the wire during the electroerosion by, for example, detecting the increase in unevenness by an optical method, in particular by measuring its specular reflectance (Japanese patent application published as No. 54-71496) or by a mechanical method, by means of a sensor element (Japanese patent application published as No. 59-219119). But these methods do not allow for the twisting movement of the wire which has the effect of continuously changing the orientation of the deteriorated zone and thus interferes with the monitoring measurements.
Other known methods consist in monitoring the decrease in the cross-section of the wire by measuring its resistivity (Japanese patent application published as No. 59-102 529) or its magnetic characteristics (Japanese patent application published as No. 60-29 222).
However, since variations in resistivity and magnetic effects can also be caused by a variation in the temperature of the wire and not by a change in its cross-section, the interpretation of the results of these measurements remains very uncertain, and does not make it possible to foresee or prevent effectively the break of the wire.
The present invention has as its object a device intended to prevent the breaking of a metal wire passing under tension between two guide elements, particularly in the case of a wire electrode stretched between the two machining heads of a machine for cutting by electroerosion and with a machining current passing through it. Thanks to this device, the progressive wear on the wire can be monitored by measuring the variation with time of its cross-section at the outlet of the machining zone. According to the device of the present invention the wire passes through an element of the type of rings with jets or an outflow gauge equipped with at least one barrel, or into a wire passed through an organ caliber of a pneumatic gauge disposed for example at the outlet of the machining zone of an EDM machine which makes it possible to measure the leakage of a fluid injected under pressure, either between the wire and the barrel or the wire and the ring, or else at the outlet of calibrated openings or of sensors arranged on either side of the wire.
This device is likewise arranged in such a way as to change automatically, when this measurement reaches a given threshold, at least one of the machining parameters the variation in which is suitable for reducing the wear on the wire.
The present invention likewise deals with a process using the device as defined above.
"Barrel" is understood to mean a calibrated opening arranged at the outlet of a leakproof case.
"Measuring the leakage" is understood to mean "measuring the pressure or the flow rate" of the fluid passing through the barrel or the ring or between the calibrated openings or the sensors arranged on either side of the wire. This fluid may be a gas or a liquid. Electroerosion preferably involves a dielectric machining liquid, but air or another gas may also be used. Thanks to an electronic circuit, it is possible for example to compare each measurement at each instant to a reference value and to cause at least one machining parameter defined below to vary in such a way as to restore the deviation between the measurement and the reference value to below a given threshold. It is likewise possible at the same time to make a measurement upstream from the machining zone and to compare the deviation between the two measurements obtained respectively upstream and downstream with a reference deviation and then, when it reaches or surpasses this reference value, to act on an appropriate machining parameter in such a way as to bring this deviation to below the reference value. These thresholds, deviations and reference values may be determined experimentally or by theoretical calculation.
The response time of these measurements is very short. The appearance of an abnormal wear may generally be detected in less than several tenths of a second. The return to normal wear conditions is generally done in less than several seconds.
Preferably a machining parameter is chosen which can be acted on rapidly and a change in which can rapidly reduce the wear on the wire. The parameter may also be the speed of passage of the wire, that is the duration of its passage through the machining zone, or else pull between the guide elements or even one of the characteristics of the injection of the machining liquids. In electroerosion it is preferable to act on the power of the electrical discharges, in particular on their frequency or their intensity. It is likewise possible to act on the machining conditions by also varying the delay period of the pulses, the duration or frequency of the sparks, the voltage at the terminals of the generator, or else control a series of short interruptions in the pulses.
The measurements thus made according to the device and the process of the present invention give a reliable indication of the reduction in the mean cross-section of the wire and thus of its degree of wear.
In contrast to the different devices and processes proposed in the state of the art, the present invention makes it possible to detect accurately the appearance of a process of abnormal wear on the wire, and this regardless of the orientation or the temperature of the latest.
The device of the present invention may assume numerous embodiments. Only certain ones will be illustrated diagrammatically as an example in the annexed drawings.